1. Technical Field
The present invention relates to a circuit and a method for removing a frequency offset in a demodulator of a wireless communication system and a communication apparatus including the circuit.
2. Description of Related Art
In general, a demodulator of a wireless communication system includes, for example, a configuration of obtaining a digital detected signal (hereinafter, referred to as a baseband signal) representing a frequency shift level by detecting a signal subject to a modulation process, such as an FSK (Frequency Shift Keying), and a frequency offset removing circuit, such as an AFC (Automatic Frequency Control) circuit, for removing a frequency offset which exists in a corresponding baseband signal. When the frequency offset exists, a level comparison between a slice level, that is, a predetermined determination threshold value and a signal level of the baseband signal may not be exactly performed. In this regard, data values indicated by the base band signal, such as consecutive “0” or “1”, is correctly determined by removing the frequency offset.
A method for removing a frequency offset has been known in the related art. See, for example, Japanese Patent Application Laid-open No. H11-298541 (Patent Literature 1). According to the above method, an amplitude average value of the baseband signal is calculated during a predetermined time period, and the amplitude average value serving as the frequency offset is removed. Further, another method for removing a frequency offset has been known in the related art. See, for example, Japanese Patent Application Laid-open No. 2009-71811 (Patent Literature 2). According to this method, an amplitude average value of a data pattern of a preamble added in a head of a received frame in a wireless communication system performing a burst communication is calculated, and the amplitude average value serving as a frequency offset.
However, as described in Patent Literature 1, in the method for removing an amplitude average value of a baseband signal during a predetermined time period as a frequency offset, when the predetermined time period is set short, tracking characteristic of the frequency offset removal from the baseband signal is increased, whereas the accuracy of the frequency offset removal is relatively low. To the contrary, when the predetermined time period is set long, the accuracy of the frequency offset removal is relatively increased, whereas the tracking characteristic of the frequency offset removal from the baseband signal is lowered. That is, there is a trade-off relation between the tracking characteristic of the frequency offset removal from the baseband signal and the accuracy of the frequency offset removal.
Further, as described in Patent Literature 2, in the method for removing an amplitude average value of a baseband signal during a predetermined time period as a frequency offset, when the offset occurs in a payload section after calculating the frequency offset from a preamble data pattern, the offset calculated from the preamble data pattern becomes a frequency offset that is not optimal to be removed, that is, there is a problem that a value of the frequency offset to be removed cannot be tracked to the extent of the payload section. Further, when an error occurs in detecting an access code before the preamble data is recognized, the preamble data pattern cannot be correctly recognized and the frequency offset cannot be calculated.
In order to solve the above-described problems, the present invention provides a circuit and a method for removing a frequency offset which can track rapidly and high-precisely the frequency offset to the extent of a payload section and remove the frequency offset and a communication apparatus including the circuit.